The scanning electron beam in a cathode ray display tube may be subject to defocusing due to the variation in distance from the electron gun to the screen as the beam scans horizontally and vertically. Such defocusing effects may be corrected by the use of dynamic focusing or DF, where an AC signal voltage which varies with deflection is superimposed on a DC, static focusing potential. Such a dynamic focusing arrangement is shown in U.S. Pat. No. 5,043,638 to Yamashita. The AC signal voltage may comprise a summation of a horizontal frequency parabolic shaped signal and a vertical frequency parabolic shaped signal. In a typical dynamic focusing system, the DC static focus potential applied to a focus electrode is adjustable, and may be about 9 kilovolts (KV). The AC signal voltage may be coupled to a focusing electrode by a capacitor. Such a DF coupling capacitor must have sufficient capacitance to couple the low frequency parabolic components of the AC signal voltage to the focus electrode. In addition, the capacitor requires a breakdown voltage rating of, for example, 15 KV. A DF coupling capacitor may be encapsulated or potted together with a resistive potential divider which generates the DC focusing potential. The DC focusing potential may be derived from either the ultor (EHT) supply voltage of, for example approximately 30 KV, or from a 1/3 tap on an integrated high voltage transformer (IHVT), which produces a voltage of, for example approximately 10 KV. However, in either case, a large value series resistor, for example 50 M.OMEGA., may be required to limit current flow during EHT arcing to the focus electrode within the tube.
FIG. 1 illustrates, in simplified form, a typical dynamic focusing arrangement where a DF signal, resulting from summation of horizontal and vertical parabolic shaped signals in a transformer T1, is coupled via a capacitor C3 to a focusing grid FG of a cathode ray tube (CRT). The wire harness between DF coupling capacitor C3, the focus potential divider 100 and the focus grid FG in the CRT, may exhibit stray capacitance, for example, on the order of 50 pF, shown as capacitor C4. The internal tube structure may introduce further shunting stray capacitance of, for example 20 pF, shown as capacitor C5. Thus, the AC coupled dynamic focusing signal is subject to attenuation by a capacitive voltage divider formed by coupling capacitor C3 and the shunting capacitance of capacitors C4 and C5.
It is desirable that such stray capacitance be minimized and well controlled, and to this end, lead dressing may control stray or distributed capacitance by spacing the leads from other conductors. However, this technique is inconvenient and may be impractical in mass production. In addition, it may not be completely effective, so as to require the DF coupling capacitor C3 to be increased in value in order to reduce the potential divider action of the stray capacitance. However, the high breakdown voltage rating requirement of the DF coupling capacitor C3 imposes a significant increase in the size and cost of the capacitance if its value is increased.
In order to compensate for the attenuation of the DF signal by the capacitive voltage division, the amplitude of the DF signal may be increased to compensate for the attenuation. However, this may require DF signal amplifiers having higher power capability, and higher breakdown voltage. In addition, transformer T1 may require a higher rated value of core saturation or corona breakdown level.
It is also desirable that a focus divider be utilized which operates coupled to a 1/3 tap on the integrated high voltage transformer (IHVT, not shown), since such a divider is widely used and does not require potting, and thus is of lower cost than an encapsulated unit. Furthermore, it would be desirable to use a DF coupling capacitor which does not need to be potted in order to prevent arcing across its electrodes.